Simplify the juliac compatible fork-exec

This patch simplifies the juliac compatible pipe-fork-exec
implementation for running git on the formatted text. Instead of
collecting all bytes and then writing to stderr we instead write
directly from the intermediate buffer to stderr. The drawback is that if
reading bytes fail we might have already written something to stderr,
but this should rarely, if ever, happen under normal circumstances.

src/juliac.jl

@@ -130,9 +130,7 @@ function run_juliac(cmd::Base.CmdRedirect)
     argv = cmd′′.exec
     dir = cmd′′.dir
     # Run the command
-    bytes = pipe_fork_exec(argv, dir)
-    # Write output
-    write(Core.stderr, bytes)
+    pipe_fork_exec(argv, dir)
     return
 end
 
@@ -145,6 +143,7 @@ end
 
 function pipe_fork_exec(argv::Vector{String}, dir::String)
     local pipe, fork, dup2, chdir, execv, waitpid # Silence of the Langs(erver)
+    STDOUT_FILENO, STDERR_FILENO = 1, 2
     # Set up the pipe
     fds = Vector{Cint}(undef, 2)
     READ_END, WRITE_END = 1, 2
@@ -161,7 +160,6 @@ function pipe_fork_exec(argv::Vector{String}, dir::String)
         err = @ccall close(fds[READ_END]::Cint)::Cint
         err == -1 && systemerror("close")
         # Duplicate write end of the pipe to stdout and stderr
-        STDOUT_FILENO, STDERR_FILENO = 1, 2
         err = @ccall dup2(fds[WRITE_END]::Cint, STDOUT_FILENO::Cint)::Cint
         err == -1 && systemerror("dup2")
         err = @ccall dup2(fds[WRITE_END]::Cint, STDERR_FILENO::Cint)::Cint
@@ -180,18 +178,24 @@ function pipe_fork_exec(argv::Vector{String}, dir::String)
     # Close write end of the pipe
     err = @ccall close(fds[WRITE_END]::Cint)::Cint
     err == -1 && systemerror("close")
-    bytes = UInt8[]
-    buf = Vector{UInt8}(undef, 1024)
+    # Shuffle bytes from the pipe to stderr
+    bufsize = 1024
+    buf = Vector{UInt8}(undef, bufsize)
     while true
-        nread = @ccall read(fds[READ_END]::Cint, buf::Ptr{Cvoid}, 1024::Csize_t)::Cssize_t
+        # Read bytes from the pipe
+        nread = @ccall read(fds[READ_END]::Cint, buf::Ptr{Cvoid}, bufsize::Csize_t)::Cssize_t
         nread == -1 && systemerror("read")
         nread == 0 && break # eof
-        append!(bytes, @view(buf[1:nread]))
+        # Write bytes to stderr
+        nwrite = @ccall write(STDERR_FILENO::Cint, buf::Ptr{Cvoid}, nread::Csize_t)::Cssize_t
+        nread == -1 && systemerror("write")
+        @assert nwrite == nread
     end
-    err = @ccall close(fds[READ_END]::Cint)::Cint # Close the read end of the pipe
+    # Close the read end of the pipe
+    err = @ccall close(fds[READ_END]::Cint)::Cint
     err == -1 && systemerror("close")
 
-    # Check exit status of the child
+    # Wait for, and check exit status of, the child
     status = Ref{Cint}()
     wpid = @ccall waitpid(cpid::Cint, status::Ref{Cint}, 0::Cint)::Cint
     wpid == -1 && systemerror("waitpid")
@@ -199,5 +203,5 @@ function pipe_fork_exec(argv::Vector{String}, dir::String)
         error("child process did not exit normally")
     end
     # crc = WEXITSTATUS(status) # ignore this like `ignorestatus(cmd)`
-    return bytes
+    return
 end